1. Field of the Invention
The present invention relates to a fuel injection control system for an engine. In particular, the present invention relates to a fuel injection system for an engine which is suitable for enhancing operability while retaining various performances such as low fuel consumption due to lean combustion in a wide range of operating conditions.
2. Description of Background Art
A lean combustion control has been known where the air-fuel ratio of a fuel-air mixture is controlled to be higher than a theoretical air-fuel ratio at the time of steady operation and the time of gentle acceleration of the engine. For example, in an aircraft reciprocating engine, the air-fuel ratio is gradually shifted to the lean side by operating a mixture control lever provided separately from a power lever for varying the throttle opening. As the air-fuel ratio is gradually shifted to the lean side, the fuel consumption performance is enhanced, but the engine begins to loose ignition when the air-fuel ratio reaches or exceeds a predetermined value. The air-fuel ratio in this instance is called a “lean limit”, and its value varies largely depending on whether the engine is of the lean combustion type or not.
FIG. 12 is a diagram showing an example of the relationship between the air-fuel ratio (corresponding to the throttle opening) and the fuel consumption rate, for a lean combustion type engine and an ordinary engine. In the case of the ordinary engine, the lean limit is present in the vicinity of an air-fuel ratio of 17. In the case of the lean combustion type engine, the lean limit is present on the leaner side. Therefore, a low fuel consumption rate is maintained even when the leanness is brought to such a point that the quantity of air cannot be increased further, by fully opening the throttle valve.
In the case of the ordinary engine, the throttle opening at the lean limit is generally set in the vicinity of an intermediate opening. In order to open the throttle valve further so as to increase the suction air quantity, the mixture control lever is manually operated together with the power lever so as to enrich the fuel-air mixture according to the output, whereby the engine output characteristics can be secured.
Such a control system for an aircraft reciprocating engine is disclosed, for example, in Japanese Patent Laid-open No. Hei 6-247392.
In the background art as above-mentioned, in order to increase the fuel injection amount after the lean limit is reached in the ordinary engine, it is necessary for the pilot to operate the mixture control lever separately from the power lever, so as to regulate the fuel injection amount. Specifically, the pilot must operate both the power lever and the mixture control lever.
Furthermore, in the background art, even in a range in the vicinity of or on the lean side of the lean limit, the engine ignition timing has been set on the basis of only the engine speed. Therefore, there has been the problem that when the air-fuel ratio is shifted to the lean side by a lean combustion control, it is difficult to achieve ignition in the engine at an optimum timing.